Detonation interrupter

ABSTRACT

A disruptor ( 10 ) has a plastics barrel ( 12 ) cast from unreinforced polyurethane so that only plastic deformation of the barrel occurs during initiation of a cartridge ( 18 ) a discharge of a water charge ( 24 ).

This invention relates to an interrupter, also known as a disrupter. Interrupters are devices employed to disrupt explosives, particularly terrorists' explosives.

BACKGROUND

Interrupters (or disruptors) are distinct from dearmers, at least in the context of this patent specification. Both dearmers and interrupters function by driving a projectile into munitions, disrupting the detonation process so that the explosives in the munitions are not detonated before the detonation apparatus is destroyed by the action of the projectile.

In the case of military munitions, the explosives are almost invariably encased in metal such as steel so that the dearmer itself needs to be powered by explosive or propellant means with sufficient force in order to penetrate the casing. Indeed, with dearmers in relation to military munitions, the personnel wishing to disarm the munitions often know its design. This means that solid projectiles, directed specifically at the detonation apparatus or fuse of the munitions are often beneficial.

However, in the case of terrorist bombs (Improvised Explosive Devices, IEDs), these are invariably an unknown quantity, but often not usually encased in a steel jacket. Therefore interrupters do not use a solid projectile and water is most usually the projectile employed. High velocity solid projectiles may be detrimental in this case, since the pressure generated by their impact may inadvertently cause detonation of the (unknown) explosive material. The propellant is therefore generally a deflagrating charge that expands rapidly but which produces a less intense driving impetus than a detonating charge. Water spreads after it has been fired, and on impact with any target. Therefore if the exact location of the detonation apparatus is unknown within the package of munitions, there is a still a high probability of the projectile being able to disrupt the target.

Even without an explosive charge, however, the rapid burning of the propellant in an interrupter accelerates the projectile with significant force. Consequently, a current design employs a steel tube, closed at a breech end that houses a propellant cartridge. A water projectile is disposed in front of the cartridge. The interrupter is often deployed on a robotic vehicle because, with terrorist bombs, there is often a danger of remote or timed detonation that may pose a significant risk to personnel. Robotic vehicles are often small and nimble, enabling them to be deployed in many different locations where terrorist bombs may be left. However, they need to carry other equipment such as cameras so that they can be remotely steered and aimed. Consequently, disruptors generally represent a significant load for the robotic vehicle and reduction of this load is highly desirable.

GB-A-2083894 discloses a dearmer having a steel barrel, and metal or water projectile, and the arrangement is typical of that genre of apparatus, whereas GB-A-2030684 discloses a disrupter having a similar construction but wherein the projectile, being either water or a cylindrical body, is driven by expanding gases of a propellant.

U.S. Pat. No. 4,779,511 discloses a disposable dearmer which has a relatively light body, and is only so strong as to “contain the explosive long enough to expel the slug with the proper velocity”. It is suggested that glass-fibre reinforced plastics may be suitable, but accepts that the body may be damaged or destroyed on detonation.

WO-A-03/058155 discloses an explosive disruptor which is characterised in being light, indeed, made of plastics material. The plastics housing contains a precise and predetermined explosive charge and a projectile. The plastics casing disintegrates on detonation and the projectile, as well as the explosive charge can be selected and sized to suit the target. However, although light, the complete disintegration of the housing and the explosive charge poses a significant risk of damaging a robotic vehicle, and surrounding infrastructure.

It is an object of the present invention to provide a disruptor that is light but does not pose a significant risk to its surroundings when fired.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present invention, there is provided a disrupter comprising

-   -   a barrel     -   a breech mechanism at a breech end of the barrel to receive a         cartridge of deflagrating propellant;     -   a charge of liquid,     -   seals in the barrel to prevent the liquid charge leaking from         the muzzle end of the barrel and from contacting the cartridge;         wherein     -   the barrel is cast in a mould from plastics material whereby,         when the cartridge is initiated to drive the charge from the         barrel, the material of the barrel is distorted plastically but         is not fractured.

By not being fractured is meant that expanding propellant gases from the initiated cartridge do not escape through the barrel wall, but serve only to drive the charge along the barrel, subject to any loss of energy through such plastic deformation of the barrel wall.

The barrel wall, at least in the vicinity of the cartridge, must be thick enough so that the plastics material can withstand the strains imposed by initiation and subsequent expansion of the propellant gasses. The surprising element is how thin the wall can be and yet be strong enough to withstand such strains, at least on one occasion, in any event. The absolute thickness depends on the nature of the plastics material, as well as the strains imposed by the cartridge.

Polyurethane is a suitable material. For example, a barrel approximately 400 mm long, inner diameter 27 mm and wall thickness 8 mm has a mass of around 450 g, and can withstand a propulsive pressure generated at the breech end of in excess of 500 bar applied for several milliseconds. This is sufficient to propel a mass of water of 100-150 g with sufficient velocity to form an effective disruptor jet. The barrel may undergo significant plastic deformation but does not fail. It is important that the barrel does not fail, both to retain the propulsive pressure in order to produce a jet of sufficient velocity, and to eliminate fragment hazards which may cause significant collateral damage.

Polyurethane, with a static failure strength of between 30-50 MPa, has been found to perform well. A barrel of such dimensions, exposed to a static internal pressure would undoubtedly fail at a pressure well below 500 bar. Surprisingly, calculations, based on the static strength of the material, show that the stresses generated in the barrel by the application of the dynamic pressure of 500 bar over a few milliseconds, indicate that stresses well in excess of 50 MPa are experienced by the material. Without being tied to any particular theory, it appears that some form of, as yet unquantified, dynamic enhancement of the material strength occurs when the barrel is loaded at such high rates, and that this enhancement enables the material to deform plastically without failing.

Accordingly, unreinforced plastics material can surprisingly be employed. At least, the plastics material may be unreinforced by any thing that is not poured with the plastics material into the mould.

The mould may include a thread for connection of a breech cap.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is further described hereinafter, by way of example, with reference to the accompanying drawing, in which FIG. 1 is a schematic illustration of a disrupter in accordance with the invention.

DETAILED DESCRIPTION

In FIG. 1, a disruptor 10 has a tubular barrel 12 of cast polyurethane. The barrel is approximately 400 mm long, has and inner diameter of 27 mm and a wall thickness 8 mm. It is screw threaded at a breech end 14 to receive a breech 16. An electrically fired deflagration cartridge 18 is disposed inside the breech end 14 of the barrel, within the confines of the breech 16. The cartridge may be such as employed by a disruptor currently marketed under the brand Pigstick by A B Precision (Poole) Limited (see website at http://www.abprecision.co.uk/pdf/pigstick.pdf for further detail). In any event, the cartridge employs a deflagrating propellant that does not explosively detonate but which burns rapidly on initiation producing rapidly expanding gases. An air-filled space 20 is provided between the cartridge 18 and a piston 22, which space is pressurised by the expanding gases of the deflagrating propellant on initiation thereof to a pressure in excess of 500 bar. Beyond the piston is a charge of water retained by an end cap 28 which is a sealing, but otherwise loose fit, in the discharge end 26 of the barrel 12.

It is a feature of the present invention that the barrel is not reinforced, since, although this might well strengthen the barrel, it also reduces its ductility, rendering it brittle. It is the ductility that on the one hand contains the rapid increase in pressure, while on the other, the dynamic strength of the material appears much greater at the strain rates imposed by the cartridge propellant deflagration than its published static strength suggests.

The barrel 12 is cast from molten polyurethane or similar plastics material in an open mould (not shown). The mould has an internal thread to form a corresponding thread on the barrel. The barrel is therefore unscrewed from the mould once the plastics material has cooled and solidified.

Consequently, there are low assembly costs so that, despite being disposed of after each detonation, the cost of a new barrel is not significant. The breech 16 may also be made from polyurethane or other suitable plastic. In the current use an injection moulded polyethylene breech is used such that it does not require machining of its thread 30 on its internal bore or wire apertures 32 for the firing wires (not shown). The breech may or may not be reused a number of times.

Overall the arrangement is light and can be carried by a small robotic vehicle. Furthermore, since it does not fragment on firing, there is no danger of collateral damage to the vehicle. The current design has a total mass of less than 650 g compared with re-usable steel barrels and breeches which typically weigh in excess of 2.5 kg for a similar performance.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 

1-17. (canceled)
 18. A disruptor comprising a barrel a breech mechanism at a breech end of the barrel to receive a cartridge of deflagrating propellant; a charge of liquid, seals in the barrel to prevent the liquid charge leaking from the muzzle end of the barrel and from contacting the cartridge; wherein the barrel is cast in a mould from plastics material whereby, when the cartridge is initiated to drive the charge from the barrel, the material of the barrel is distorted plastically but is not fractured.
 19. A disruptor as claimed in claim 18, in which the plastics material is a polyurethane.
 20. A disrupter as claimed in claim 18, in which the plastics material has a static failure strength of between 20 and 60 MPa.
 21. A disrupter as claimed in claim 20, in which the plastics material has a static failure strength of between 30 and 50 MPa.
 22. A disrupter as claimed in claim 18, in which the barrel is between 200 and 600 mm long, has an inner diameter of between 20 and 40 mm and a wall thickness of between 5 and 15 mm.
 23. A disrupter as claimed in claim 22, in which the barrel is between 300 and 500 mm long, has an inner diameter of between 25 and 35 mm and a wall thickness of between 7 and 12 mm.
 24. A disriptor as claimed in claim 23, in which the barrel is between 380 and 420 mm long, has an inner diameter of between 26 and 30 mm and a wall thickness of between 7 and 9 mm.
 25. A disrupter as claimed in claim 18, in which the barrel has a mass of between 200 and 600 g.
 26. A disrupter as claimed in claim 25, in which the barrel has a mass of between 300 and 500 g.
 27. A disrupter as claimed in claim 26, in which the barrel has a mass of between 380 and 420 g.
 28. A disrupter as claimed in claim 18, in which the barrel withstands a propulsive pressure generated at its breech end of between 500 and 700 bar applied for up to 10 milliseconds.
 29. A disrupter as claimed in claim 18, in which the liquid charge has a mass of between 100 and 150 g.
 30. A disruptor as claimed in claim 18, in which the plastics material is unreinforced by any thing that is not poured with the plastics material into the mould.
 31. A disrupter as claimed in claim 18, in which the barrel has a cast thread for connection of a breech cap.
 32. A method of making a disrupter, comprising the steps of casting a barrel from thermoplastic polymer in mould; cooling the polymer so that it solidifies and removing it from the mould; inserting a deflagrating propellant cartridge at a breech end of the barrel; connecting on a breech cap; inserting a water projectile, and providing electrical connections to the cartridge for firing same.
 33. A method as claimed in claim 16, in which the mould is provided with an internal thread formation at one end of the mould, said removing step including unscrewing the barrel from the mould; and said connecting of the breech cap comprises screwing it onto the thread on the barrel. 